This invention relates to lithographic printing. In particular, this invention relates to positive working, multi-layer thermally imageable elements in which the top layer comprises a crosslinked polymer.
In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plates, also called printing plate precursors, typically comprise a top layer applied over the surface of a hydrophilic substrate. The top layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
If after exposure to radiation, the exposed regions are removed in the developing process, revealing the underlying hydrophilic surface of the support, the plate is called as a positive-working printing plate. Conversely, if the unexposed regions are removed by the developing process and the exposed regions remain, the plate is called a negative-working plate. In each instance, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution, and repel ink.
Direct digital imaging of offset printing plates, which obviates the need for exposure through a negative, is becoming increasingly important in the printing industry. Positive working, multi-layer, thermally imageable elements that comprise a hydrophilic substrate, an alkali developer soluble underlayer, and a thermally imageable top layer have been disclosed. On infrared exposure, the exposed regions of the top layer become soluble in or permeable by the alkaline developer. The developer penetrates the top layer and removes the underlayer and the top layer, revealing the underlying substrate. Such systems are disclosed in, for example, Parsons, WO 97/39894 and U.S. Pat. No. 6,280,899; Shimazu, U.S. Pat. No. 6,294,311; Nagasaka, EP 0 823 327; Miyake, EP 0 909 627; West, WO 98/42507; and Nguyen, WO 99/1145.
Despite the advantages that have been made in the development of multi-layer thermally imageable elements, elements in which the top layer has increased resistance to developer and to damage during handing would be desirable. Thus, a need exists for positive working, multi-layer, thermally imageable elements that have increased resistance to developer and to damage during handing, but whose imaging speed is unaffected.
In one aspect, the invention is a positive working, multi-layer, thermally imageable element, the imageable element comprising, in order:
a substrate having a hydrophilic surface,
an underlayer comprising a first polymeric material over the hydrophilic surface, and
a top layer comprising a second polymeric material over the underlayer, in which:
the second polymeric material is crosslinked;
the top layer is ink receptive and insoluble in an alkaline developer;
the top layer and the underlayer are each removable by the alkaline developer following thermal exposure of the element; and
the element comprises a photothermal conversion material.
In another aspect, the invention is a method for forming the element. In another aspect, the invention is a method for forming an image by exposing and developing the element. In yet another aspect, the invention is an image, useful as a lithographic printing plate, formed by exposing and developing the element.